Water Rocket Forum

Welcome to the Water Rocket Forum, sponsored by The Water Rocket Achievement World Record Association.

The largest, most sophisticated and ground breaking group supporting you, the serious water rocket flyer! Whether you are a beginner or an expert, the WRA2 has something for everyone.

A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel (the engine of the rocket) is constructed from thin plastic or other non metallic materials (usually a used plastic soft drink bottle) weighing 1,500 grams or less. The water is forced out by compressed air. It is an example of Newton's third law of motion.

air.command wrote:
I too agree with what you are saying. This was the reason I mentioned the desire to have a hollow launch tube. ARBs (Air Reservoir Boosters) on the launcher have been used for many years. They supply extra air up the launch tube to compensate for the increase in volume inside the rocket. This is why simulators include launcher volume as a parameter. Robert Youens used a 30L ARB on his launcher. http://txsnapper.eezway.org/txsnapper/insaneair.html

There are also other small effects that determine the difference between hollow and solid tubes such as the behaviour of the water inside the rocket during launch.

U.S. Rockets1 and air.command, both of your posts make a lot of sense. I would have to build a larger volume launcher in order for the pressure drop while the rocket is still on the launch tube in order for the pressure deviation/drop is minimized. Though, this would require a more time pumping air into the system-launcher and rocket. I would have to get a better bike pump or an electric one if I want to accomplish this. I could see this being another analysis-making modular PVC launchers with varying volumes and determining pressure-volume displacement ratio. PVC is fairly inexpensive too... I am not sure what kind of plot that would be of Pressure Deviation/Drop (pressure while cabled down - pressure before nozzle leaves launch tube) versus Volume of Launcher (while cable-tied down) but I am sure I could spend a few hours at some point figuring that out. I believe it would just be a matter of using an ideal gas law equation (P_1*V_1 = P_2*V_2) and simple geometric calculations to find volume; though, I am not sure what the temperature change is when the pressure drops-not sure if that is significant enough to include. (Not sure what's up with the posting on the forums but I thought I posted this and somehow did not-disregard this if it looks like a repost from your guys' computer)

air.command wrote:
I too agree with what you are saying. This was the reason I mentioned the desire to have a hollow launch tube. ARBs (Air Reservoir Boosters) on the launcher have been used for many years. They supply extra air up the launch tube to compensate for the increase in volume inside the rocket. This is why simulators include launcher volume as a parameter. Robert Youens used a 30L ARB on his launcher. http://txsnapper.eezway.org/txsnapper/insaneair.html

Thanks for the link.

I saw another design recently using a elevated farm/lawn sprinkler with a large bbq propane bottle attached.

Looked like a fun setup that would easily be filled/managed and supply a little extra kick.

In regards to the launcher here found here (http://www.uswaterrockets.com/construct ... torial.htm), how is the vertical portion of the launcher able to stay up? I see you have the U-bolts fastened down to the PVC fittings but do the bumps on the fittings able to secure it strongly with a fueled T6 - T10 FTC rocket sitting on the launch tube? That is one issue with my designing aspect since the majority of the launch tube is not reinforced structurally but within the FTC; a higher center of gravity than I would feel comfortable with...

It is actually not hard to calculate the force that a cable tie launcher is capable of holding.

If you have a cable tie rated for 100 pounds, and you put 32 of them around your rocket nozzle then you have a system capable of holding 3200 (100 * 32) pounds of force. If you have a nozzle with 1 square inch of opening area then you could hold a rocket pressurized to 3200PSI. Since most rockets use smaller nozzles than this at lower pressures, then you can see that the average rocket will never cause a cable tie launcher to fail.

I see what you are saying, but how the manufacturer rates the load and tests is at question.

Is it applied with stretching action along the length of the tie, or perpendicular and with shear force, or at one small focal point of a .050 x .250 x .250 area. not to mention what shear force the clamp holding the ties, or the bottle neck flange can handle.

If you have realized these pressures with the ties, then that's really cool, but the assembly as a whole in actual use sounds like the extreme limits for all plastics involved. Armchair conjecture on my behalf, at full speed here. Respectfully.

The ratings for fasteners must be understated to allow for a safety margin. In reality the fasteners will support about 1.5x the rating. The amount of stretching should be very small at anything below the rated pressure.

We never really tried to measure the stretching under extreme pressure because it never caused a launch to fail. If we had encountered problems we would have collected measurements to correct using the data, but it just worked.

I haven't built a cable tie launcher yet. But when I do, I will make it a point to test the strengh of the cable ties I intend to use. I have some Autocraft brand cable ties that are only good for bundling wires. I can pull them apart with my hands.